Perform unused assignment and unused variables lints on MIR.

[cjgillot]

Based on #101399
Based on #102256
Blocked on #108993

Rough attempt to #51003.

This is my first stab at using the dataflow framework, so please be gentle 😄

In order to keep the same level of diagnostics, I had to instrument MIR a little more:

• introduce a no-op fake read kind for let _ = syntax. This changes the behaviour of let _ = <access to unsafe field> currently type-checks #54003, I don't know if it's still desirable.
• keep for which original local a guard local is created;
• store in the VarBindingForm the list of introducer places and whether this was a shorthand pattern.

I am not very proud of the handling of self-assignments. The proposed scheme is in two parts: first detect probable self-assignments, by pattern matching on MIR, and second treat them specially during dataflow analysis. I welcome ideas.

I did not attempt to rewrite the dead-code lint, as #100288 does it.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras

[rust-highfive]

r? @davidtwco

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[rustbot]

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[bors]

☔ The latest upstream changes (presumably #108944) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[RalfJung]

I guess this is also based on #104844, or where does this diff come from? (#104844 is not listed in the PR description.)

[cjgillot]

Yes. Added it to the description.

[cjgillot]

Actually, it doesn't need to be. I'll remove that commit in a few.

[bors]

☔ The latest upstream changes (presumably #109303) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #109092) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #96840) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #106934) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

Some changes might have occurred in exhaustiveness checking

cc @Nadrieril

[bors]

☔ The latest upstream changes (presumably #113646) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

@cjgillot any updates on this

[Dylan-DPC]

oh nvm this was blocked

[rustbot]

Some changes occurred in match lowering

cc @Nadrieril

[Nadrieril]

Could you document what "shorthand" means?

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

## Running ui tests in tests/pass for x86_64-unknown-linux-gnu
   Compiler: "MIRI_ENV_VAR_TEST"="0" "MIRI_TEMP"="/tmp/miri-uitest-c103fq" "RUST_BACKTRACE"="1" /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/bin/miri "--error-format=json" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--target" "x86_64-unknown-linux-gnu" "--out-dir" OUT_DIR

FAILED TEST: tests/pass/align_repeat_into_well_aligned_array.rs
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-c103fq" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage1/bin/miri" "--error-format=json" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--target" "x86_64-unknown-linux-gnu" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/ui/tests/pass" "tests/pass/align_repeat_into_well_aligned_array.rs" "-Zmiri-symbolic-alignment-check" "--edition" "2021"
error: pass test got exit status: 1, but expected 0

error: actual output differed from expected
Execute `./miri test --bless` to update `tests/pass/align_repeat_into_well_aligned_array.stderr` to the actual output
Execute `./miri test --bless` to update `tests/pass/align_repeat_into_well_aligned_array.stderr` to the actual output
--- tests/pass/align_repeat_into_well_aligned_array.stderr
+++ <stderr output>
+error: variable `a` is assigned to, but never used
+   |
+LL |     let mut a = Params::new();
+   |         ^^^^^
+   |
+   |
+   = note: consider using `_a` instead
+   = note: `-D unused-variables` implied by `-D unused`
+   = help: to override `-D unused` add `#[allow(unused_variables)]`
+error: value assigned to `a` is never read
+  --> $DIR/align_repeat_into_well_aligned_array.rs:LL:CC
+   |
+   |
+LL |     a.key_block = [0; BLOCKBYTES];
+   |
+   |
+   = help: maybe it is overwritten before being read?
+   = note: `-D unused-assignments` implied by `-D unused`
+   = help: to override `-D unused` add `#[allow(unused_assignments)]`
+error: miri cannot be run on programs that fail compilation
+
+error: aborting due to 3 previous errors
+
---

error: there were 1 unmatched diagnostics
##[error] --> tests/pass/align_repeat_into_well_aligned_array.rs:6:9
  |
6 |     let mut a = Params::new();
  |         ^^^^^ Error: variable `a` is assigned to, but never used

error: there were 1 unmatched diagnostics
##[error] --> tests/pass/align_repeat_into_well_aligned_array.rs:9:5
  |
  |
9 |     a.key_block = [0; BLOCKBYTES];
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Error: value assigned to `a` is never read

full stderr:
full stderr:
error: variable `a` is assigned to, but never used
   |
LL |     let mut a = Params::new();
   |         ^^^^^
   |
   |
   = note: consider using `_a` instead
   = note: `-D unused-variables` implied by `-D unused`
   = help: to override `-D unused` add `#[allow(unused_variables)]`
error: value assigned to `a` is never read
##[error]  --> tests/pass/align_repeat_into_well_aligned_array.rs:9:5
   |
   |
LL |     a.key_block = [0; BLOCKBYTES];
   |
   |
   = help: maybe it is overwritten before being read?
   = note: `-D unused-assignments` implied by `-D unused`
   = help: to override `-D unused` add `#[allow(unused_assignments)]`
error: miri cannot be run on programs that fail compilation

error: aborting due to 3 previous errors

---
Error: 
   0: ui tests in tests/pass for x86_64-unknown-linux-gnu failed
   1: tests failed

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
error: test failed, to rerun pass `--test ui`
Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/deps/ui-f4b6be260663ac84 --quiet` (exit status: 1)
Build completed unsuccessfully in 0:02:54
  local time: Sun Apr  7 19:22:39 UTC 2024

[bors]

☔ The latest upstream changes (presumably #123569) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

Closing this as it's been blocked on another pr for a long time and that pr is in draft state and hasn't moved and this one has bitrotted.